1. Field of the Invention
The present invention relates to a purification catalyst for exhaust gas, to a production method therefor, and to a purification catalyst device for automobile exhaust gas, and specifically relates to a production technique for a purification catalyst for exhaust gas in which nitrogen oxides (NOx), carbon hydride (HC), and carbon monoxide (CO) contained in an exhaust gas emitted from an internal combustion engine of an automobile or the like during operation can be simultaneously and effectively purified, whereby the amount of exhaust gas is reduced.
2. Related Art
For purifying exhaust gas containing, for example, CO, HC, NO, and NO2, precious metals (Pt, Rh, Pd and Ir) exhibit high performance. Therefore, it is preferable to employ the above-mentioned precious metals in a purification catalyst for exhaust gas. These precious metals are generally mixed with or supported by Al2O3 which is a support material having high surface-to-weight ratio together with additives such as La, Ce, and Nd. On the other hand, composite oxides (for example, a perovskite oxide), made by combining various elements, have extremely varied properties. Therefore, it is preferable for a purification catalyst for exhaust gas to employ the above-mentioned composite oxides. Moreover, when the precious metal is supported on the composite oxides, the properties of the precious metal are significantly changed. From this point of view, a preferable performance for purifying exhaust gas can be obtained in the purification catalyst for exhaust gas in which a precious metal is supported on a composite oxide.
Various catalysts mentioned above are now being developed, and for example, a technique in which the rate of coalescence of the precious metal can be reduced by making a perovskite be a support, judging from deterioration of the precious metal with reduction of active sites by coagulation of the precious metal, is proposed (see Japanese Unexamined Application Publication No. 5-86259). Moreover, another technique in which reduction of PdO can be decreased by using a perovskite in which the A site is defective, in view of reducing PdO which is an active species in a NO reduction reaction, whereby the PdO is changed to Pd, which is low-activity Pd, when the precious metal is Pd, is proposed (see Japanese Unexamined Application Publication No. 2003-175337).
Conventional purification catalysts for exhaust gas exhibit satisfactory performance for reducing CO, HC, and NOx (NO, NO2, etc.) contained in exhaust gas, in the initial running of a vehicle. However, the conventional catalysts cannot exhibit satisfactory performance for reducing CO, HC, and NOx, after running for long periods or an exposure to high temperature conditions.
As mentioned above, a reason that sufficient performance for purifying the exhaust gas cannot be obtained after running for long periods or an exposure to high temperature conditions is as follows. That is, in the conventional purification catalyst for exhaust gas, a precious metal, for example, Pt, Rh, or Pd, is supported on Al2O3 having a high surface-to-weight ratio. Due to the high surface-to-weight ratio of the Al2O3, the precious metal is advantageously supported in a highly dispersed condition. However, Al2O3 is a stable compound and does not mutually affect a supported precious metal, whereby activity of the precious metal is not improved. This is why the Al2O3 has an extremely stable surface, whereby Al2O3 does not electrically affect and interact with the precious metal. Accordingly, satisfactory performance cannot be obtained after running for long periods or exposure to high temperature conditions.
Moreover, the reason that endurance of the Al2O3 is low is as follows. That is, decreases in the activity thereof occur due to decrease of active sites with the coagulation of the precious metals, specifically Pd based on the deterioration form of the catalyst containing the precious metals, and by degradation and reduction of PdO whose oxide state is highly active. Due to the extremely stable surface of the Al2O3, Al2O3 does not have an effect of decreasing the mobility of Pd for the precious metal during the running of an automobile, and does not have an effect of stabilizing the oxide state of the PdO at the surface, whereby the decreasing of the activity cannot be suppressed.